In this study, the effect of pilot arrangement on OFDM system performance for different time and frequency selective fading channels is investigated. Pilot symbols are placed in rectangular 2-D pilot pattern in chosen intervals both in time and frequency domain, and corresponding system performances are given in terms of BER. The pilot symbols that give the lowest BER for different channel profiles are determined as the optimum pilot symbols, and the interval between these optimum pilot symbols are defined as the optimum pilot interval in time and frequency direction. Then the relationships between the optimum pilot intervals and channels' coherence times and coherence bandwidths are evaluated. It is seen from the results the optimum pilot interval can be related best with coherence time and coherence bandwidth at 0.9 correlation. Therefore 2.5 pilot symbols per Bc,0.9 and 25 pilot symbols per Tc,0.9 can be used. © 2012 IEEE.